It's really common to include extra attributes in a web page,
for assorted browser-compatibility reasons. The extra tags
are ignored by browsers that don't understand, and everyone is happy.
Except the validator. It would nice to have a little button that says
"show only structual errors". The validator would check the site,
but allow extra tags and extra attributes to tags.
But any misuse of a known tag or attribute would be flagged.
I'd like to be able to check real-world web pages, not just showcases
to compatibility with the HTML specification.
-Bryce